WO2008131964A2

WO2008131964A2 - A thermal store

Info

Publication number: WO2008131964A2
Application number: PCT/EP2008/003506
Authority: WO
Inventors: Nicholas David Beckett
Original assignee: Heat Energy And Associated Technology Limited
Priority date: 2007-05-01
Filing date: 2008-04-30
Publication date: 2008-11-06
Also published as: CN101939595A; EP2150753A2; US20100263606A1; GB0708420D0; WO2008131964A3; EA200901462A1

Abstract

A thermal store system for a water heating system comprising a primary thermal store containing a primary volume of water adapted to be heated by a primary heating means, said primary thermal store being in thermal communication with a central heating supply flow and a central heating return flow, a secondary water heat exchanger in thermal communication with the primary volume of water, the secondary water heat exchanger having a water feed flow and a water outlet flow for supplying hot water, at least one of said central heating return flow and/or said water feed flow being in thermal communication with at least one further thermal store, upstream of said primary thermal store, said at least one further thermal store containing a medium adapted to be heated by at least one supplementary heating means.

Description

A THERMAL STORE

The present invention relates to a thermal store for a domestic or commercial heating and hot water supply. Thermal stores are well known in the art and their mode of operation is now described. A thermal store uses a volume of primary water heated directly by a boiler powered generally by fossil fuels. This is typically faster and more efficient than indirect methods of water heating. Indirect water heating typically requires multiple boiler firings to achieve satisfaction of the thermostat demand. In a single cylinder thermal store the contained volume is primary heated water. The pumped circulation creates a well-mixed heated volume from one boiler firing which is fast and efficient. In the thermal store direct hot water is then generated by passing mains pressure cold water through a heat exchanging circuit, located in the upper stratification of the volume of primary heated water. The primary volume of water is typically heated to a temperature above the design flow temperature of the central heating system and a standard thermostat controls this. It is considered wise to stipulate minimum set temperature of 65°C to obviate any concerns over Legionella. Supplied direct hot water temperatures are then regulated using an externally mounted TMV (Thermostatic Mixer Valve) connected between the circuit output and the mains pressure cold water circuit. The TMV at the store ensures minimum energy depletion and significantly reduces any risk of scalding. For reference, current Legionella related legislation insists that the output temperature is not less than 50⁰C. If the DHW is supplied via a loop the return temperature must not be less than 50⁰C.

The central heating circuit output temperature can also be regulated using a TMV which again minimises energy depletion from the store while ensuring delivery of the central heating flow at design temperature. This thermal regulation is essential for low temperature systems or underfloor systems requiring less than the declared 65°C minimum storage temperature, and typically 40-50⁰C. The installation of a thermal store permits the removal of system header tanks from the roofspace. Direct hot water is at mains pressure offering optimum flow rates and ensuring a full and balanced supply for thermostatic shower systems. Potable water is not stored either as a cold volume or hot volume reducing the health risks attributable to both. It is desirable to reduce reliance on fossil fuels, due to increasing cost and environmental concerns, in particular in relation to CO2 emissions, by supplementing heat supplied from a primary oil or gas fired boiler with heat from one or more renewable or at least more efficient and/or less polluting secondary heat sources by providing one or more heat exchanger coils within the thermal store receiving heated water from a secondary source. For example, it is desirable to be able to supplement the primary heat supply with solar derived heat energy. However, because of the relatively high temperature of the thermal store , such secondary sources can only contribute heat to the thermal store when the temperature of the fluid heated by the secondary source exceeds the temperature of the fluid in the thermal store. With regard to solar heating, such temperatures can only be achieved under ideal circumstances (achievable as hot sunny days), and thus solar heating cannot contribute heat to the thermal store during much of the year.

It is an object of the present invention to provide an improved thermal store in order to improve the efficiency of thermal stores by optimising the ability of the thermal store to receive heat from secondary, preferably renewable or low emission heat sources, thus mitigating the effects of global warming by reducing green house gas emissions.

According to the present invention there is provided a thermal store system for heating water, comprising a primary thermal store containing a primary volume of water adapted to be heated by a primary heating means, said primary thermal store being in thermal communication with a central heating supply flow and a central heating return flow, a secondary water heat exchanger in thermal communication with the primary volume of water, the secondary water heat exchanger having a water feed flow and a water outlet flow for supplying domestic hot water, said central heating return flow and/or said water feed flow being in thermal communication with at least one further thermal store, upstream of said primary thermal store, said at least one further thermal store containing a medium adapted to be heated by at least one supplementary heating means for supplying heat to said central heating return flow.

Preferably both said central heating return flow and said water feed flow are in thermal communication with said at least one further thermal store whereby said flows receive heat by thermal communication with said at least one further thermal store before receiving heat from said primary thermal store.

The primary heating means may be powered by fossil fuel. Preferably the primary thermal store is provided with a first heat exchange circuit coupled to said primary heating means for heating the primary volume of water.

Preferably said primary volume of water is in fluid communication with said central heating supply flow and said central heating return flow whereby said primary volume of water forms part of a central heating circuit.

Said at least one further thermal store may be adapted to be heated by two or more supplementary heating means.

Said primary thermal store may be provided with one or more further water heat exchange circuits adapted to receive heat from one or more further sources of heat.

Said at least one further thermal store may be provided with two or more heat exchange circuits, each heat exchange circuit being adapted to receive heat from a further source of heat.

Said at least one supplementary heating means may comprise at least one of a heat pump, an electrical immersion heater, a solar energy collector, a biomass heat source a fuel cell or any other available heat source.

Two or more further thermal stores may be provided, each further thermal store containing a medium to be heated by a respective supplementary heating means, said two or more further thermal stores being connected in series whereby the central heating return flow and/or the water feed flow is heated from a low temperature to a high temperature through heat exchange with the medium in each successive thermal store. In one embodiment said medium contained in said at least one further thermal store comprises a secondary volume of water. To save costs, said further thermal store may have substantially the same structure as the primary thermal store.

Preferably said further thermal store is provided with a heat exchange circuit adapted to receive heat from said supplementary heating means for heating the secondary volume of water. The secondary volume of water is preferably in fluid communication with said central heating supply flow and said central heating return flow whereby said secondary volume of water forms part of a central heating circuit.

Preferably said central heating return flow enters the further thermal store at or adjacent a lower region of the further thermal store and exits said further thermal store at or adjacent an upper region of the further thermal store. A central heating circuit completing conduit may be provided between said primary and further thermal stores providing fluid communication between an upper region of the further thermal store and a lower region of said primary thermal store. Preferably said circuit completing conduit is provided with a non-return valve to prevent reverse flow between the primary and further thermal stores. A flow communication may be provided between said primary thermal store and said further thermal store, said flow communication being provided with a pump for selectively passing water between said primary and further thermal stores when the temperature of said primary volume of water is less than the temperature of the secondary volume of water.

Preferably the water supplied to said water feed means passes through a water heat exchanger in thermal communication with said secondary volume of water before entering said secondary heat exchanger of said primary thermal store. Preferably said water heat exchanger comprises a first heat exchange coil provided in a lower region of the further thermal store and a second heat exchanger coil, connected downstream of said first heat exchange coil, provided in an upper region of the further thermal store, said first heat exchange coil generating a depletion zone in the lower region of the further thermal store. In an alternative embodiment, said medium contained in said at least one further store comprises a material adapted to change between solid and liquid states at substantially the operating temperature of the supplementary heating means adapted to supply heat to said medium. Such materials are commonly referred to as "Phase Change Materials" or PCMs. Said medium may comprise a mixture or combination of PCMs adapted to melt at different tempertures falling within a predetermined range of the operating temperature of the respective supplementary heating means. Two or more further thermal stores may be provided, each further thermal store containing said medium, said two or more further thermal stores being connected in series whereby the central heating return flow is heated from a low temperature to a high temperature through heat exchange with the medium in each successive thermal store.

Preferably said central heating return flow and/or said water feed flow passes through a heat exchange circuit in each of said two or more further thermal stores in turn to receive heat from the medium contained in each subsequent further store before passing into said primary thermal store.

Preferably the two or more further thermal stores are arranged such that the operating temperature of supplementary heating means of each further thermal store increases in the direction of flow of the central heating return flow from a relatively low temperature to a relatively high temperature to provide heat transfer between the central heating return flow and the medium of each further thermal store in turn.

In one embodiment , the secondary heat exchange circuit comprises a solar thermal collector couplable to a solar circuit heat exchanger within the second container.

Preferably, the solar circuit heat exchanger is a heat exchanging coil.

Ideally, the third heat exchange circuit comprises a heat exchanging coil in thermal communication with the primary water and being adapted to receive heat from one of a biomass, electric boiler, or other available thermal energy source.

Ideally, the secondary water temperature is regulated using an externally mounted TMV (Thermostatic Mixer Valve) connected between the water outlet means and the water feed means. The water feed means is preferably a mains pressure cold water supply.

Advantageously, the TMV on the secondary water supply ensures minimum energy depletion and significantly reduces any risk of scalding.

Preferably, the central heating circuit output temperature is regulated using a TMV. Advantageously, energy depletion from the store is minimised while ensuring delivery of the central heating flow at design temperature.

Ideally, the primary thermal store has a primary water thermostat for controlling the temperature of the primary water in therein via at least the primary heating circuit.

Preferably, the primary water's thermostat is set to a temperature dependent on the design heating output or to a minimum of 65°C.

Ideally, the primary thermal store has a maximum temperature thermostat for safety and for prevention of over heating.

Preferably, the secondary heat exchange circuit has a secondary heat exchange circuit control thermostat. Ideally, the third heat exchange circuit has a third heat exchange circuit control thermostat.

Preferably, the further similar thermal store has the same thermostats, primary heat exchange circuit and secondary water heat exchanger as the first thermal store.

Ideally, the primary heat exchange circuit of the further thermal store is coupled to a heat pump.

Ideally, the central heating water return of the primary thermal store is coupled to the further thermal store.

Preferably, the central heating water return of the primary thermal store is coupled at or about the bottom of the further thermal store. Ideally, the water outlet means of the secondary water heat exchanger of the further or second thermal store is coupled to the water feed means of the secondary water heat exchanger of the primary thermal store. Ideally, a hydraulic circuit-completing conduit is coupled between the second thermal store and the primary or first thermal store.

Preferably, the hydraulic circuit-completing conduit is coupled between the upper region of the second thermal store and the primary heat exchange circuit return of the first thermal store. Advantageously, this completes the hydraulic circuit created by the passage of water flowing from the central heating water return of the first thermal store to the container of the second thermal store. This circuit connection may benefit from the addition of a non-return valve to prevent any back flow from the first to the second thermal store, while the pumped boiler circuit is not active. Ideally, the secondary water source is couplable to an input of the heat-exchanging coil of the secondary heat exchange circuit of the second thermal store.

Preferably, the output of the heat-exchanging coil of the secondary heat exchange circuit of the second thermal store is couplable to the input means of the secondary water heat exchanger of the first thermal store. A crossflow conduit may be coupled between the first thermal store and the second thermal store. The crossflow conduit may be provided with an associated crossflow pump.

Preferably, the crossflow conduit is coupled between the lower regions of both thermal stores.

Ideally, direct heating from one of a wind turbine and / or a PV (photo voltaic) input is provided to the primary and/or secondary volume of water.

Preferably, the micro-wind and PV energy is supplied via direct immersion elements. The direct immersion elements may be either DC or AC depending on the equipment specification.

Ideally, heating elements are provided in either or both thermal store Preferably, the heating elements may be switched to optimise performance.

In one embodiment of the invention, the water feed means of the secondary water heat exchanger of the primary thermal store is coupled to one or more auxiliary heat stores utilising phase change materials. Preferably , the central heating return means of the thermal store is thermally coupled to the one or more auxiliary heat stores.

The one or more auxiliary heat stores may comprise a heat pump heat store with phase change at or around 45°C coupled in series to a solar phase change heat store with phase change at or around 62⁰C.

In this, the phase change store deployed may have three circuits. One 'charging' path and two depletion paths. The charge circuit of a heat pump store is connected across the heat pump flow and return heating circuit. One discharge circuit is coupled to the crossflow circuit of the thermal store and may also receive the heating return circuit. The second discharge circuit is coupled to the cold water feed. The discharge circuits of the lower temperature heat pump store are connected in series with the discharge circuits of the higher temperature solar heat store. The solar store charging circuit is coupled to the solar collection means, flow and return.

The cold feed discharge circuit is coupled to the input side of the secondary heat exchanger of the thermal store. The other discharge circuit is coupled to the primary return to the thermal store. Advantageously, this completes the hydraulic circuit created by the passage of water flowing from the central heating water return of the thermal store to the heat pump heat exchanger.

In such embodiment, the heat pump heat store may have one or more thermostats for controlling the operation of the heat pump.

The solar source heat store may be provided with one or more thermostats for controlling the operation of the solar circuit.

A crossflow conduit may be coupled between the thermal store and the heat pump heat exchanger and has an associated crossflow pump, preferably coupled between the lower region of the thermal store and the heat pump heat exchanger.

In such embodiment, the heat pump heat exchanger is capable of providing significant latent energy release at or around 45°C. The store can be elevated to a maximum temperature of 90°C. The solar source heat exchanger may be capable of providing significant latent energy release at or around 62°C. The store can be elevated to a maximum temperature of 90⁰C.

A plurality of heat pump heat exchangers may be coupled together. Advantageously, this arrangement may be utilised when it is beneficial to collect and store larger amounts of energy for depletion via the thermal store.

In one embodiment a plurality of solar source heat exchangers may be coupled together.

When thermal stores are used on an industrial scale, the thermal stores are required to hold 1000 to 2000 litres which is significantly more than the 210 or 300 litres , typically required for domestic purposes.

The invention will now be described with reference to the accompanying drawings which show by way of example.. In the drawings:-

Figure 1 is a single hybrid thermal store; Figure 2 is a dual hybrid thermal store in accordance with a first embodiment of the present invention; and

Figure 3 is a dual hybrid thermal store in accordance with a second embodiment of the present invention.

Referring initially to Figure 1, there is shown a hybrid thermal store indicated generally by the reference numeral 1 having a container 2 for storing a primary volume of water 3. The thermal store 1 has a primary heating circuit 4 coupled to a primary heating source (not shown) such as a fossil fuel, biomass or wood pellet boiler via a primary heating circuit flow pipe 5 and a primary heating circuit return pipe 6 for heating the primary volume of water. A pump (not shown) pumps the primary water to and from the primary heating source. The container 2 has a central heating flow pipe 8 and a central heating return pipe 9 and a secondary water heat exchanger 11 in thermal communication with the primary volume of water 3. The secondary water heat exchanger 11 has water feed pipe 14 and a water outlet pipe 15. The thermal store 1 has a secondary heat exchange circuit 16 couplable to a secondary heating source (not shown) and the secondary heat exchange circuit 16 is disposed below the secondary water heat exchanger 11 within the container 2. The thermal store 1 is adaptable to be coupled to a second similar thermal store to define a dual hybrid thermal store, as shown in Figure 2. The thermal stores 1 coupled together are substantially identical. The thermal store

1 has a third heat exchange circuit 21 couplable to a third heating source, (not shown). The third heat exchange circuit 21 is disposed intermediate the secondary heating circuit 16 and the secondary water heat exchanger 11 within the container 2 of the thermal store 1. For a single thermal store configuration (Fig 1) the secondary heating circuit 16 may be solar powered and the secondary heating circuit 16 may comprise a solar thermal collector coupled to a solar heat exchanger 23 within the container 2.

The third heat exchanger circuit 21 comprises a heat exchanging coil 25 in thermal communication with the primary water 3 and being couplable to a biomass, wood pellet or electric boiler or other thermal energy source (not shown). Advantageously, when the second heating circuit 16 and/or the third heating circuit 21 are connected, then these heat sources make a contribution to both central heating and hot water. This is not a possibility using a standard indirect twin coil cylinder, currently commonplace in the market. By installing the hybrid thermal store 1 there is ready access available for the addition of a secondary heat input and or third thermal input, at any time, without any need for draining the system.

The secondary water temperature is regulated using an externally mounted thermostatic mixer valve TMV 31 connected between the water output pipe 15 and the water feed pipe 14. The water feed pipe 14 is preferably connected to the mains pressure cold water circuit. Advantageously, the TMV 31 on the secondary water supply ensures minimum energy depletion and significantly reduces any risk of scalding. The central heating circuit output temperature is also regulated using a TMV 33 coupled between the central heating flow pipe 8 and the central heating return pipe 9. Advantageously, energy depletion from the thermal store 1 is minimised while ensuring delivery of the central heating flow water at design temperature.

The container 2 has a primary store thermostat 35 for controlling the temperature of the primary water 3 in the container 2 via the primary heating circuit 4. The primary store's thermostat 35 is set to a temperature dependent on the type of heating source used by the primary heating circuit 4. The container 2 has a maximum temperature thermostat 36 for control input and safety to prevent over-heating. The secondary heating circuit 16 has a secondary heating circuit control thermostat 37 and the third heating circuit 21 has a third heating circuit control thermostat 38. Referring now to Figure 2, there is shown a hybrid thermal store in accordance with a first embodiment of the invention, indicated generally by the reference numeral 50 wherein a second similar thermal store 51 is coupled to a first thermal store 1 similar to that shown in Figure 1. The primary heating circuit 4 of the second thermal store 51 is coupled to a heat pump. The second thermal store 51 has the same structural configuration as the first thermal store 1 and in particular the second thermal store 51 has the same container 2, the same thermostats 35, 36, 37 and 38, the same heating circuits 16 and 21 and secondary water heat exchanger 11 as the first thermal store 1. Components of the second thermal store 51 corresponding to equivalent components of the first thermal store 1 have therefore been given identical reference numerals. The central heating water return pipe 9 of the first thermal store 1 may be coupled to a lower region of the container 2 of the second thermal store 51 at or about the bottom of the container 2 of the second thermal store 51. The water outlet pipe 15 of the secondary water heat exchanger 11 of the second thermal store 51 is coupled to the water feed pipe 14 of the secondary water heat exchanger 11 of the first thermal store 1. Thermostatic mixer valve 31 of the first thermal store 1 has a direct cold water feed 81. A hydraulic circuit-completing conduit 61 is coupled between the upper region of the second thermal store container 2 and the first thermal store 1. The conduit 61 may contain a non-return valve (not shown) to avoid the risk of reverse flow. The hydraulic circuit-completing conduit 61 is coupled between the upper region of the second thermal store container 2 and the primary heating circuit return pipe 6 of the first thermal store 1. Advantageously, this completes the hydraulic circuit created by the passage of water flowing from the central heating water return pipe 9 of the first thermal store 1 to the container 2 of the second thermal store 51.

The secondary water source pipe 62 is coupled to an input pipe of the heat- exchanging coil 23 of the secondary heating circuit 16. The output pipe 64 of the heat- exchanging coil 23 of the secondary heating circuit 16 is coupled to the input pipe of the secondary water heat exchanger 11. A crossflow conduit 72 is coupled between the first thermal store 1 and the second thermal store 51 and has an associated crossflow pump 71 mounted thereon. The crossflow conduit 72 is coupled between the lower region of both thermal stores 1 , 51.

Combining the energy available from heat pump systems has proved difficult for a number of reasons, primarily because the typical flow temperatures for optimum efficiency are in the region of 50⁰C and there is a consequent need to provide and control energy 'top up' in order to achieve the required 65°C. Technology developments are current whereby heat pump systems may be able to efficiently deliver water at 65⁰C. If and when this is achievable the need for energy 'top up' is removed and the consequent requirement for fossil fuel input further minimised. The connection of a second hybrid thermal store 51 , as shown in Figure 2, enables the addition of a heat pump primary heating circuit 4, either ground or air source. The addition of the second thermal store 51 provides for the doubling of stored capacity, with beneficial effects for both heat pump and solar thermal performance. As both thermal stores 1 , 51 are substantially identical there is a beneficial impact on cost based on volume manufacture as well as inventory and logistics. The configuration may be controlled using standard thermostatic control technology or more advanced electronic alternatives, as applicable. In use, referring in the main to Figure 2, thermal store 1 has its primary store thermostat 35 set at or above 63°C. This calls for primary heating circuit 4 support once depleted, using the primary energy source. Thermal store 51 is connected with the heat pump as the primary source, with the primary store thermostat 35 set below the maximum maintainable level for the heat pump, nominally 48°C. The solar thermal heating circuit 16 uses a standard differential controller regulating input to thermal store 51. This dual hybrid thermal store configuration 50 favours energy depletion from thermal store 51 , thereby holding off use of the fossil fuel input to thermal store 1. The dual hybrid thermal store configuration 50 is designed such that in all operating modes energy is drawn from thermal store 51. This enables a priority contribution from solar thermal ensuring optimal input conditions. The second priority enables the heat pump via standard thermostatic control technology. Depending on the electricity tariff structures available, the heat pump primary circuit 4 can be enabled constantly or a timed regime is provided, if beneficial electricity supply rates are available. Alternatively the provision of a timer can facilitate the avoidance of premium supply rates. Advanced software based electronic control systems may be employed to optimise the energy collection.

With the heat pump primary circuit 4 enabled, the minimum fully charged temperature for thermal store 51 is 50⁰C. With the thermal store 51 in this state, hot water draw-off causes cold water (6-10⁰C typical) to flow through secondary water source pipe 62 and through the heat exchanging coil 23 taking a small amount of pre-heat energy from the container base generating a depletion zone in the proximity of the flow for the heat pump primary circuit 4 and the solar thermal inputs, if connected to the third heating circuit 21 of the second thermal store 51. The secondary water source then passes through the secondary water heat exchanger 11 at the top of the second thermal store 51 , thereby extracting significant energy there from (approximately 35-42°C depending on flow rate). The flow continues to the secondary water feed pipe 14 of the first thermal store 1. The energy depletion from first thermal store 1 is then equivalent to the difference between the incoming secondary water flow temperature of 35-42°C and the level deliverable by the secondary water heat exchanger 11 of the first thermal store, again depending on the flow rate but in the region of 55-60⁰C. The energy depletion from thermal store 1 is now approximately 60% less than required from a stand-alone system.

Similarly when the heat pump of the primary heating circuit 4 of the second thermal store 51 is switched on the central heating flow 8 is extracted from the first thermal store 1, If the heating system is designed to deliver return temperatures of 40⁰C or less the return central heating flow 9 is directed to the depletion zone in the second thermal store 51. The hydraulic circuit is completed by a conduit 61 connecting the top of second thermal store 51 with the primary heating circuit return pipe 6 of the first thermal store 1. In the current worst- case example central heating water pumped out at 65°C is replenished by 50⁰C. Ideally, for optimum performance the maximum design central heating return water temperature should equate to the maximum output temperature from the heat pump. There is a significant efficiency benefit from a reduction in central heating flow temperature that may be compensated by an increase in design flow / return differential. The minimum design energy state is determined by the heat pump capability. The maximum energy state is achieved via solar thermal input and/or direct heating from a micro-wind turbine / PV (photo voltaic) input. The micro-wind or PV energy can be supplied via direct immersion elements either DC or AC depending on the equipment specification. Heating elements can be provided in either or both thermal stores 1 , 51 and can be switched to optimise performance. Depending on the type of solar thermal collection specified temperatures of 80-90⁰C are achievable with prolonged exposure and collection. The control system is configured to allow both thermal stores 1 , 51 to achieve a maximum temperature of 90°C.

Once the temperature in the second thermal store 51 exceeds 65°C use of domestic hot water and central heating begins to transfer energy from thermal store 51 to thermal store 1 whereby thermal store 51 is depleted allowing for replacement energy to be stored up to the maximum capacity. The maximum system capacity becomes both thermal stores 1 , 51 charged to 90⁰C, detected by thermostats 36. A differential thermostat controller is connected to both thermal stores 1 , 51 measuring the temperature difference between the temperature of the two thermostats 36 at both locations. When the temperature of thermal store 51 exceeds the temperature of thermal store 1 by 2°C or more and the temperature recorded by the thermostat 36 in thermal store 1 is >90°C and the heating pump is not running, the additional cross-flow pump 71 is energised. This ensures that thermal store 1 remains at an equivalent charge level to thermal store 51 ensuring the energy storage potential is maximised at all times.

Referring now to Figure 3, there is shown a second embodiment of the invention, wherein a thermal store 151 of identical structure and function to the thermal store 1 or 51 of Figure 2is shown with the water feed 14 of the secondary water heat exchanger 11 of the thermal store 151 coupled to an auxiliary heat store indicated generally by the reference numeral 152. The central heating return 9 of the thermal store 151 is coupled to the auxiliary heat store 152.

The auxiliary heat store 152 comprises a phase change heat pump heat store 154 coupled in series to a phase change solar source heat store 156, each comprising a vessel containing a material adapted to change between solid and liquid states at substantially the operating temperature of the heat source to which the vessel is in thermal communication. The heat pump heat store 154 has a heat pump flow and return heating circuit 157, a central heating return receiving port 159 coupled to the central heating return 9 of the thermal store 151 and a cold feed receiving port 161 coupled to a cold water feed such as the mains. The heat pump heat exchanger 154 has an arrangement for communicating water from the heat pump heat exchanger 154 to the solar source heat exchanger 156.

In one embodiment, each auxiliary heat store contains a mixture or blend of materials, each having a different melting point within a range around the operating temperature of the heat source from which the respective auxiliary heat store receives heat. The arrangement for communicating water from the heat pump heat exchanger 154 to the solar source heat exchanger 156 is a pair of separate water carrying conduits 163 and 164. The heat pump heat exchanger 154 has two thermostats 166 / 168 for controlling the operation of the heat pump. The heat pump heat exchanger

The solar source heat exchanger 156 has a solar source flow and return heating circuit 171 , an arrangement for receiving water from the heat pump heat exchanger 154 and an arrangement for delivering water to the thermal store 151. The arrangement for receiving water from the heat pump heat exchanger 154 comprises two ports 173, 174 for receiving the water carrying conduits 163, 164 extending from the heat pump heat exchanger 154. The arrangement for delivering the water to the thermal store 151 comprises a conduit 176 coupled between the solar source heat exchanger 156 and the water feed pipe 14 of the secondary water heat exchanger 11 of the thermal store 151.

The solar source heat exchanger 156 has two thermostats 177 / 178 for controlling the operation of the solar source.

A hydraulic circuit-completing conduit 181 is coupled between the solar source heat exchanger 156 and the thermal store 151. The hydraulic circuit-completing conduit 181 is coupled between the upper region of the solar source heat exchanger 156 and the primary heating circuit return 6 of the thermal store 151. Advantageously, this completes the hydraulic circuit created by the passage of water flowing from the central heating water return 9 of the thermal store 151 to the heat pump heat exchanger 154.

A crossflow conduit 172 is coupled between the thermal store 151 and the heat pump heat exchanger 154 and has an associated crossflow pump 271. The crossflow conduit 172 is coupled between the lower region of the thermal store 151 and the heat pump heat exchanger 154. The heat pump heat exchanger 154 is capable of providing a water temperature up to approximately 42°C and the solar source heat exchanger 156 is capable of providing a water temperature up to approximately 62°C. In a commercial arrangement, (not shown) a number of heat pump heat exchangers are coupled together which number could be above one hundred and comprising a variety of phase change temperatures. Advantageously, this arrangement may be utilized when a larger volume of water is required for a commercial/industrial thermal store container with a large capacity. In this arrangement, the same number of solar source heat exchangers are also coupled together. The quantity of the coupled store arrays is determined by the design energy collection capability. Either energy source may be used independently although the combination of both can be considered thermally complementary. When thermal stores are used on an industrial scale, the thermal stores are required to hold up to 1000 to 2000 litres which is significantly more than the 210 or 300 litres used for domestic purposes.

In use, cold water is pulled into cold feed receiving port 161 of the heat pump heat exchanger 154 by demand on the water outlet pipe 15 of the secondary water heater 11 of the thermal store. The storage medium in the heat pump heat exchanger 154 is maintained at a temperature in the region of 50⁰C, with a phase change point at or around 42°C. Energy is transferred via the water passing through the heat pump flow and return heating circuit 157, the operation of which is controlled by thermostat 166. Water from the heat pump heat exchanger 154 passes out along conduits 163 and 164 extending from the top of the heat pump heat exchanger 154. The water flowing along these conduits 163, 164 enters the base of the solar source heat exchanger 156 and the water is further heated by heat transfer from water passing through the solar source heat exchanger 156 via the solar source flow and return heating circuit 171 which is controlled by the thermostat 177. The storage medium in the solar source heat exchanger 156 may be heated to 90°C with a phase change point at or around 62°C. This water flows from the top of the solar source heat exchanger 156 via conduit 176 to the water feed pipe 14 of the secondary water heat exchanger 11. Clearly, the advantage of having the water pre-heated to this temperature is that there is no demand on the fossil fuel primary heating circuit 4 of the thermal store 151. Furthermore, water at elevated temperature is also passing into the return pipe 6 of the fossil fuel primary heating circuit 4 of the thermal store 151. Additionally, the central heating return pipe 9 extends into the base of the heat pump heat exchanger 154 further reducing the demand on the thermal store 151. Variations and modifications can be made without departing from the scope of the invention described above.

Claims

1. A thermal store system for a water heating system comprising a primary thermal store containing a primary volume of water adapted to be heated by a primary heating means, said primary thermal store being in thermal communication with a central heating supply flow and a central heating return flow, a secondary water heat exchanger in thermal communication with the primary volume of water, the secondary water heat exchanger having a water feed flow and a water outlet flow for supplying hot water, at least one of said central heating return flow and/or said water feed flow being in thermal communication with at least one further thermal store, upstream of said primary thermal store, said at least one further thermal store containing a medium adapted to be heated by at least one supplementary heating means.

2. A system as claimed in claim 1 , wherein both said central heating return flow and said water feed flow are in thermal communication with said at least one further thermal store whereby said central heating return flow and said water feed flow receive heat by thermal communication with said at least one further thermal store before entering into thermal communication with said primary thermal store.

3. A system as claimed in any preceding claim, wherein the two or more further thermal stores are arranged such that the operating temperature of supplementary heating means of each further thermal store increases in the direction of flow of the central heating return flow from a relatively low temperature to a relatively high temperature to provide heat transfer between the central heating return flow, and/or the water feed flow, and the medium of each further thermal store in turn.

4. A system as claimed in any preceding claim, wherein the primary heating means is powered by fossil fuel.

5. A system as claimed in claim 3, wherein the primary thermal store is provided with a first heat exchange circuit adapted to receive heat from said primary heating means for heating the primary volume of water.

6. A system as claimed in any preceding claim, wherein said primary volume of water is in fluid communication with said central heating supply flow and said central heating return flow whereby said primary volume of water forms part of a central heating circuit.

7. A system as claimed in any preceding claim wherein said at least one further thermal store is adapted to be heated by two or more supplementary heating means.

8. A system as claimed in any preceding claim, wherein said primary thermal store is provided with one or more further water heat exchange circuits adapted to receive heat from one or more further sources of heat.

9. A system as claimed in any preceding claim, wherein said at least one supplementary heating means comprises at least one of a heat pump, an electrical immersion heater, a solar energy collector, a biomass heat source, a fuel cell or any other available heat source.

10. A system as claimed in any preceding claim, wherein two or more further thermal stores are provided, each further thermal store containing a medium to be heated by a respective supplementary heating means, said two or more further thermal stores being connected in series whereby the central heating return flow and/or the water feed flow is heated from a low temperature to a high temperature through heat exchange with the medium in each successive thermal store.

11. A system as claimed in any preceding claim, wherein said medium contained in said at least one further thermal store comprises a secondary volume of water.

12. A system as claimed in claim 11 , wherein said further thermal store has substantially the same structure as the primary thermal store.

13. A system as claimed in claim 11 or claim 12, wherein said further thermal store is provided with a heat exchange circuit adapted to receive heat from said supplementary heating means for heating the secondary volume of water.

14. A system as claimed in any of claims 11 to 13, wherein said secondary volume of water is in fluid communication with said central heating supply flow and said central heating return flow whereby said secondary volume of water forms part of a central heating circuit.

15. A system as claimed in claim 14, wherein said central heating return flow enters the further thermal store at or adjacent a lower region of the further thermal store and exits said further thermal store at or adjacent an upper region of the further thermal store.

16. A system as claimed in claim 15, wherein a central heating circuit completing conduit is provided between said primary and further thermal stores providing fluid communication between an upper region of the further thermal store and a lower region of said primary thermal store.

17. A system as claimed in claim 16, wherein said circuit completing conduit is provided with a non-return valve to prevent reverse flow between the primary and further thermal stores.

18. A system as claimed in any of claims 11 to 17, wherein a flow communication is provided between said primary thermal store and said further thermal store, said flow communication being provided with a pump for selectively passing water between said primary and further thermal stores when the temperature of said primary volume of water is less than the temperature of any other connected volume of water.

19. A system as claimed in any of claims 11 to 17, wherein the water supplied to said water feed means passes through a water heat exchanger in thermal communication with said secondary volume of water before entering said secondary heat exchanger of said primary thermal store.

20. A system as claimed in claim 19, wherein said water heat exchanger comprises a first heat exchange coil provided in a lower region of the further thermal store and a second heat exchanger coil, connected downstream of said first heat exchange coil, provided in an upper region of the further thermal store, said first heat exchange coil generating a depletion zone in the lower region of the further thermal store.

21. A system as claimed in any of claims 1 to 10, wherein said medium contained in said at least one further store comprises a material adapted to change between solid and liquid states at substantially the operating temperature of the supplementary heating means adapted to supply heat to said medium.

22. A system as claimed in any of claims 1 to 10, wherein said medium contained in said at least one further store comprises a mixture or combination of materials adapted to change between solid and liquid states at different temperatures falling within a predetermined range of the operating temperature of the supplementary heating means adapted to supply heat to said medium.

23. A system as claimed in claim 21 or claim 22, wherein two or more further thermal stores are provided, each further thermal store containing said medium, said two or more further thermal stores being connected in series whereby the central heating return flow is heated from a low temperature to a high temperature through heat exchange with the medium in each successive thermal store.

24. A system as claimed in claim 23, wherein said central heating return flow and/or said water feed flow passes through a heat exchange circuit in each of said two or more further thermal stores in turn to receive heat from the medium contained in each subsequent further store before passing into said primary thermal store.

25. A thermal store system for heating water, said system comprising a first and one or more further thermal stores, each thermal store containing a medium adapted to be heated by a heating means, the heating means of a first thermal store having an operating temperature generally higher than that of the heating means of the or each further thermal store; a flow path being provided through each thermal store from the last further thermal store to the first thermal store in turn whereby a flow of water to be heated passing along said flow path sequentially receives heat from each thermal store from a relatively low temperature to a relatively high temperature.

26. A thermal store system for heating water, the system comprising a plurality of thermal stores, each thermal store having a charge circuit, in thermal communication with a respective heating means for heating a medium contained therein, and a discharge circuit for providing heat transfer between said medium and a fluid passing though said discharge circuit, whereby the discharge circuit of the thermal stores are connected in series such that a fluid flowing therethrough receives heat from each thermal store from a relatively low temperature to a relatively high temperature.